The intact and effective DNA damage response (DDR) is essential for the maintenance of genomic stability and it acts as a critical barrier to suppress tumorigenesis. We have recently identified BRIT1/MCPH1 as a novel key regulator in DDR pathway. Importantly, my lab has recently generated the BRIT1 knockout mice and demonstrated the essential roles of BRIT1 in homologous recombination DNA repair and in maintaining genomic stability in vivo. We also found significant decreases of BRIT1 gene copy number and its RNA and protein expression in multiple breast cancer lines. Significantly, we identified a BRIT1 deletion mutation in one of the 10 breast cancer specimens and this deletion led to truncation of BRIT1 protein and impaired its function in DNA damage response. Therefore, BRIT1 may function as a novel tumor suppressor for breast cancer via preserving genome stability, and targeting BRIT1 deficiency may provide novel and effective treatment for the breast cancer patients. Three specific aims are proposed to test this hypothesis: (1) To determine the effects of BRIT1 deficiency on mammary tumor formation using the BRIT1 knockout mouse. By crossing our unique BRIT1-/- mice with the MMTV-Ras transgenic mice, our initial study shows that loss of BRIT1 potentiates the Ras-induced mammary tumor development. We will compare the mammary tumor incidence, tumor grade/type and metastatic potential between MMTV-Ras/BRIT1-/- and MMTV-Ras/BRIT1+/+ mice by histological analysis. The underlying mechanisms will be investigated by analyzing the DNA repair function of major BRIT1 targets. In addition, we will determine if BRIT1 deficiency accelerates irradiation or carcinogen-induced mammary tumor development using BRIT1-conditional knockout mice. (2) To identify BRIT1 aberrations in clinical breast cancer specimens. We will identify BRIT1 aberrations from180 breast cancer samples stratified by tumor grade and HER2/ER/PR status. BRIT1 mutations in the coding region and exon/intron junction will be determined by DNA sequencing. The protein expression and subcellular location of BRIT1 will be also assessed by immunohistochemical staining, and its RNA level will be assessed using quantitative RT-PCR. In addition, we will determine if BRIT1 deficiency is correlated with HER2/ER/PR status and patient survival. (3)To develop novel treatment for BRIT1-deficient breast cancers using synthetic lethality approach. Our initial study shows that BRIT1-deficient cells are very sensitive to PARP inhibitors. To test if PARP inhibitors can serve as a potent drug targeting BRIT1-deficient breast cancers, we will systematically assess the response of the breast cancer cells to several potent PARP inhibitors in cell culture. Two mouse models carrying BRIT1-deficient mammary tumors will be used to further evaluate the efficacy of PARP inhibitors in vivo. We will also combine these inhibitors with clinic anti-cancer agents to establish the optimal therapeutic remedies. In summary, our study will contribute to an improved understanding of the key pathological alterations in breast cancer development and will provide the immediate clinic impact on the treatment of BRIT1-deficient breast cancer.